Study On The Advanced Light-Trapping Strategies And Mechanisms For Single Micro/Nanowires Solar Cells

The micro/nanowires have been widely used in the light-emitting diodes,thermoelectric devices and photovoltaic,etc,due to their prominent advantages of the ultra-small size,highly compact integration,and strong absorption capacity.Especially in the field of photovoltaic,under their unique light-trapping properties and photoelectric conversion capability,micro/nanowires could obtain higher optical absorption and photoconversion efficiency by using less material.Further improved optical absorption can be realized by advanced light-trapping design,which not only increase the photoconversion efficiency,but also enable the new application.Based on single micro/nanowires solar cell system,this thesis focuses on the modeling,simulation and design of single nanowire solar cells(SNSCs)by using finite-element method on the platform of COMSOL Multiphysics.Two novel and highly effective light-trapping designs were proposed for single micro/nanowires.The underlying mechanisms of the performance improvement as well as the optoelectronic performances were then analyzed.The core research contents and observations of this thesis are listed as follows:Combining the light-trapping mechanism of optical black hole(OBH)with SNSCs,a novel OBH SNSCs was proposed.The influences of the size of OBH,the material matching condition of the core and shell,the stepping refractive-index profile,etc,on the absorption efficiency and light trapping of OBH SNSCs are investigated.It is found that the absorption efficiency of the OBH system can be further improved by using the matching design based on the refractive index,rather than the permittivity matching.An omnidirectional and broadband absorption can be achieved when applying the OBH for micro/nano photovoltaic systems.Further study indicates that the light absorption of the OBH SNSCs can be greatly improved by optimal design of the refractive index distribution and the core radius of the OBH shell.The photocurrent of the optimize design can be enhanced up by?200%compared to bare one at the core radius is 1?m.After analyzing the strong optical resonance effect between nano-sized semiconductors and metals,we have introduced metal materials into solar cells,and proposed the design of two silver blocks on both sides of a silicon nanowire.We demonstrate that integrating a silicon single nanowire into a metallic slit can dramatically enhance the absorption efficiency over almost the whole spectral band due to the strengthened optical antenna effect.Especially,it is found that by using finite-size metallic blocks to form a nanoscale metallic cavity,the light-trapping performance of the SNSCs can be further improved.Besides,the simulation results also indicate that the effective optical absorption can be controlled by adjusting the distance of the two metal blocks and the length of the metal block.When the width of the metal block is 200 nm and the block width is 850 nm,the photocurrent density of the SNSCs coupled with the designed metallic cavity can be up to 10.4 mA/cm~2,which is enhanced by 44.4%than that of the conventional bare SNSCs.